Turbomolecular pumps are part or the nigh speed vacuum pumps which are most widely used. Turbomolecular pumps assure an oil vapor-free pumping technology due to their selective pump behavior. If, however, a turbomolecular pump is stopped, oil vapors and other impurities can travel from the fore-vacuum side to the high or hard vacuum side and can there cause considerable contamination, which is noticed in a very disadvantageous manner as an initial contamination when the pump is switched on again.
The contamination in particular, hydrocarbons and water, entail a considerable lengthening of the pump-down time. This can be prevented by flooding or venting the turbomolecular pump with an inert gas after it is stopped. Therein, the inner surfaces of the pump and the container are covered by the inert gas whereby the pump-down time is considerably shortened after the pump is switched again. Apart from that the venting or flooding causes a reduction of the coasting time of the rotor. This is criteria which is particularly important in pumps running on magnetic bearings in view of the lack of friction.
Turbomolecular pumps with venting or flooding devices are described in the DE-PS 18 09 902 and in the Journal "Vacuum Technology" (20th year 1971) volume 7, page 201 and following.
It is evident from these publications, that it was possible to establish several venting conditions in an optimal manner, in order to achieve as short a pump-down time as possible after the pump has been switched on again. An important problem has, however to date, been treated in an extremely unsatisfactory manner. This applies to the venting rate. The statement of the problem herein is the velocity with which the venting or flooding gas can be introduced into the pump, in order to further improve the optimum venting conditions, and in order to largely avoid disadvantageous effects.
The above-described problem exists, in particular, in pumps which run on magnetic bearings. The criteria which must be considered in this case, above all is the long coasting time which is caused by the lack of friction and the limitation of external forces acting upon the rotor.
The question of the venting rate is, to begin with, reduced to the cross section of the valve aperture. With a small cross section, meaning, if the pump is vented too slowly, there results an excessive coasting time of the rotor, which in most cases, can no longer be justified. With a large cross section, the forces which act upon the rotor, due to the gas flowing in, are so high that critical situations can arise where, for instance, the rotor comes into contact with the emergency bearing due to overloading the axial bearing. With average cross sections, the two disadvantageous effects overlap so that no satisfactory solution can be achieved by varying the cross section of the valve aperture. It has to be added thereto, that with the same constructional size of a pump, containers having greatly differing volumes can be closed, which additionally complicates the prior determination of optimum venting conditions for a specific type of pump.
The possibility of varying the venting rate consists in opening and closing the venting valve at intervals which have been fixedly predetermined time-wise. This requires additional effort which is not justified by the results. An optimum venting process cannot be achieved even by the above, since differing envelope conditions, for instance, the container size, type and pressure of the venting gasses and the operation of the fore pump are not considered herein. Depending upon the cycling times, there results the same disadvantages as described above.
A solution is stated in the DE-OS 40 22 523. Here, an optimum venting rate is achieved in that the forces acting upon the rotor are measured and the venting valve is actuated therein by means of a control device. The opening times of the venting valve are then optimized, in such a way, that as large a gas quantity as possible is allowed to flow in, wherein the forces, which then act collectively upon the rotor, do not exceed a predetermined magnitude which is defined by safety criteria. In rotors having an actively regulated magnetic support, in most cases, a force measurement is available by means of measured values, which are, in any case, available in the regulation circuit of the magnetic bearing. In this case, the current in the electromagnets of the magnetic bearing is a direct measurement of the force.
However, several criteria still exist which have not been taken account of in the solution just described above. The optimum position of the rotor is limited by the top and bottom magnitudes of the air gaps in the axial magnetic bearing. The maximum gap is limited since, with a much larger gap, the forces in the bearing diminish, so that, beginning with a specific value, a stable position of the rotor can no longer be assured.
The forces increase with reduction of the gap. At the same time, the natural frequencies of the regulation circuit increase until they no longer can be damped out, and the regulation circuit thus becomes unstable. This defines the bottom limit for the associated air gap. Apart from that, the forces upon the rotor also increase at smaller air gaps, which forces arise due to the working motion of the regulation cycle or circuit and the always existing more or less strong or pronounced interferences or malfunctions. Thus, also the reaction forces on the pump housing increase. In a great many application cases, these vibrations of the pump housing are very annoying or even unacceptable. Therefore, in continuous operation, the gap of the magnetic bearing should remain as close as possible to the upper limit value.
During a venting process, however, when a higher load bearing capacity is specified for the magnetic bearing as has already been mentioned above, the larger forces of the magnetic bearing at a rotor position close to the bottom limit value would be advantageous. In this case, the larger vibrations would cause no trouble since the pump is coasting.